Butterfly casually mentioned this place as being a key Salvadorean eatery among the many in the Mt. Pleasant neighborhood. It is indeed very, very, popular, and it deserves its popularity. A Chowhound dive, to be sure, but expect lines and slow service (typically Salvadorean, they insist on making everything from scratch). You'll be rewarded for the wait, though. Walk up to the counter and get a number. I tried the squash rellenos, and they were topped with a wonderfully aromatic salsa. I can't wait to go back.
Consumer note: feed those meters, and feed 'em good. The cops may be hiding out in Georgetown, but parking enforcement here is anything but shy.